1. Field of the Invention
The present invention relates to an overvoltage protection apparatus and method of a high voltage inverter. More especially, the present invention relates to an overvoltage protection apparatus and method of a high voltage inverter adapted to protect a load without any adverse effect from an overvoltage aroused by voltage fluctuation attributable to an impedance difference among an inverter, a load and a cable connected between the inverter and the load.
2. Description of the Related Art
In general, inverters are much used in the case of accurately driving a load such as 3-phase electric motors. In order to drive a load using the inverter, a cable is connected to supply a voltage outputted from the inverter between the inverter and the load.
The impedances of the inverter, the load and the cable differ from each other. Hence, a voltage reflection phenomenon takes place where a voltage produced at the inverter and supplied through the cable to the electric motor reflects at a discontinuity point of the impedance, resultantly creating voltage fluctuations.
Cancellation of the produced voltage fluctuations does not have an impact on the inverter normally driving a load. However, overlap of the produced voltage fluctuations causes an over-voltage more than a pressure resistant to the load insulation applies to a load to thereby produce insulation breakage and bearing damage, etc. due to the stress accumulation in a load.
A high voltage inverter in a cascade H-bridge fashion is low in the amount of a voltage change per unit time compared to a general inverter. Therefore, in a high voltage of cascade H-bridge mode, there occurs less insulation breakage of a load and a bearing damage and also less malfunction of peripheral devices, etc. due to an over-voltage compared to the general inverter even if the length of a cable connecting it to the load is longer.
However, the use of a high voltage inverter of cascade H-bridge mode fails to completely remove voltage fluctuations, and there still occurs voltage fluctuations causing damage to a load.
The impact due to voltage fluctuations, occurs more greatly in a case of fastly switching some switching devices configuring a high voltage inverter of cascade H-bridge mode or with a shallow width of a PWM (Pulse Width Modulation) signal to switch the switching devices.